A prior art multiplexing method known as OFDM (Orthogonal Frequency Division Multiplexing) has been applied extensively to high data rate digital transmission, such as digital audio/TV broadcasting and wireless LANs. Prior art OFDM receivers must work properly under varying conditions such as speed, temperature and humidity at a reasonable cost. Since demodulation is sensitive to frequency deviations, and because frequency deviations with respect to the suppressed carrier will result in a shift in the received spectrum, a need exists to control the frequency deviations. In the prior art OFDM systems, fast Fourier transform (FFT) techniques have been used to determine the frequency deviation.
Because the above-described prior art OFDM technique is very sensitive to varying conditions and synchronization errors, a prior art method for frequency synchronization of OFDM systems has been proposed that includes a two-step process consisting of coarse synchronization and fine synchronization. Coarse synchronization compensates for a frequency offset of an integer number of the subcarrier spacing, while fine synchronization corrects for a frequency offset smaller than one-half of the subcarrier spacing. The coarse synchronization and the fine synchronization must be performed separately for the prior art frequency synchronization, because the maximum estimation range of the fine synchronization is one-half of subcarrier spacing. Examples of prior art algorithms for coarse synchronization include GIB (Guard Interval Based) and PB (Pilots Based).
Coarse synchronization can be accomplished by comparing the position of the received spectral lines with the initial reference peak positions (i.e., expected positions). Coarse synchronization provides an accuracy of 0.5 of the frequency spacing of the data carriers. However, various leakage components (e.g., unwanted harmonics) are generated when the FFT window does not fit with an integer number of periods of the received signal. Consequently, fine synchronization is required to compensate for that problem, thus requiring both coarse and fine synchronization to solve the aforementioned problems.
In Timothy M. Schmidl and Donald C. Cox, “Robust Frequency and Timing Synchronization for OFDM,” IEEE Trans. on Communication, Vol. 45, No. 12, pp.1613–1621, December 1997, the contents of which is incorporated herein by reference, Schmidl and Cox propose a prior art OFDM symbol for synchronization which repeats an identical pattern twice in a single OFDM symbol to increase the estimation range by one subcarrier spacing. Further, IEEE, Supplement to Standard for Telecommunications and Information Exchange Between Systems-LAN/MAN Specific Requirements-Part 11: Wireless MAC and PHY Specifications: High Speed Physical Layer in the 5-GHz Band, P802.11a/D7.0, July 1999, the contents of which is also incorporated herein by reference, defines the training OFDM symbol such that the repetition period is ¼ of the useful data interval, which increases the subcarrier spacing by a factor of two. However, a two-step synchronization process (i.e., coarse synchronization and fine synchronization) is still required.
The above-described two-step synchronization sequence requires a two-symbol training sequence, usually at the beginning of a frame. Each symbol is preceded by a guard interval for dealing with multipath effects, and each frame begins with a number of system symbols, including a zero symbol used for frame synchronization and to determine channel properties, and a training symbol for initial phase reference.
The symbol/frame timing is found by searching for a symbol, where the first half is identical to the second half in the time domain. Then, the carrier frequency offset is corrected according to the prior art coarse and fine frequency synchronization. As noted above, two symbols are required in the prior art system to estimation the frequency offset, and each symbol has two halves, with a portion of each training symbol copied from the first half to the second half, as illustrated in FIG. 8.
FIG. 8 illustrates a signal architecture for a wireless local area network (WLAN) according to the prior art OFDM system, according to the IEEE Supplement. A guard interval G1, G2, G3, G4, G5 is provided at the beginning of each training symbol R1, R2, R3, R4 an data symbol D1. The first training symbol R1 is used for signal detection and gain control, the second training symbol R2 is used for fine and coarse frequency synchronization, the third training symbol R3 is used for timing synchronization and the fourth training symbol R4 is used for channel estimation. Then, the data symbols D1 follow. For example, in each of the symbols, the guard interval is N/4, where N=64, such that the length of the guard symbol is 16. For the first and second training symbols, the pattern will repeat 10 times, in the manner as noted above and in Schmidl and Cox.
FIG. 9 illustrates a second prior art data structure according to T. Keller and L. Hanzo, “Orthogonal Frequency Division Multiplexing Synchronization Techniques for Wireless Local Area Networks,” Proc. Of PIMRC '96, pp. 963–967, 1996, which is incorporated herein by reference. A null symbol N0 having no signal is provided as the first symbol, and is followed by a first training symbol R1 for timing and coarse and fine frequency synchronization, followed by a second training symbol R2 for channel estimation, and then the data symbols D1. A guard interval G is provided at the beginning of each of the data symbols and the second training symbol. However, the null symbol NO and the first training R1 symbol do not have a guard interval G.
FIG. 10 illustrates a prior art OFDM system transmitter and receiver for the prior art data architectures illustrated in FIG. 8 and FIG. 9. Fine synchronization occurs before FFT at B, and coarse synchronization occurs after FFT at A. After fine synchronization occurs at B, the guard interval G is removed from each symbol by counting to detect each symbol's starting point, and the remaining symbols without the guard interval are subjected to a serial to parallel converter, and then the FFT. Next, the symbols go through coarse frequency synchronization at A, and are further processed to yield a serial data output at the receiver.
FIG. 11 illustrates the timing and fine frequency offset estimation of the prior art OFDM system at B of FIG. 10. After an analog-to-digital conversion (ADC) 51, a frequency offset for the fine synchronization is accomplished by a delayer 53 and conjugator 55 mixed with the output ADC signal at a mixer 57. A moving sum 59 is then calculated and output to the frequency offset calculator 61 and timing offset calculator 63. The maximum value of the timing offset is then detected, and the frequency offset is calculated in accordance with the timing offset estimation. However, additional coarse synchronization is required in the prior art OFDM system after the timing and frequency offset have been estimated in the fine synchronization, as illustrated in FIG. 10 at A.
The prior art OFDM system and method has various problems and disadvantages. For example, the estimation ranges are insufficient to overcome the need for both the coarse and fine synchronization steps. Further, the prior art OFDM system must process all of the subcarriers for timing and frequency synchronization. As noted above, both coarse and fine synchronization are required to synchronize the frequency. Further, the timing offset estimation depends on the frequency offset estimation.
The prior art OFDM system must also repeat the same pattern twice in one OFDM symbol to increase the estimation range by 1 subcarrier spacing. For example, ¼ of the useful data interval is required to increase subcarrier spacing by 2. However, these estimation ranges are also insufficient to overcome the need for both the coarse and fine synchronization steps. Increasing the interval has the additional disadvantage of decreasing sum length and accordingly, decreasing performance.